Typical multiple frequency band (multiband) antennas have one part of the antenna active for one band, and another part active for a different band. A multiband antenna may have lower than average gain or may be physically larger than equivalent single band antennas. The design of antennas for mobile wireless communications are dictated by a number of factors, but mainly the volume available for the antenna, the frequency (directly related to this volume) of operation and unique environmental constraints of the wireless communication path (also related to frequency of operation), such as the distance over which wireless communication is to be performed, path loss and such like.
Antennas focus radiated RF energy in it radiation pattern such that there appears to be more power coming from the antenna in a particular direction. The electrical characteristics of an antenna, such as gain, radiation pattern, impedance, bandwidth, resonant frequency and polarization, are the same whether the antenna is transmitting or receiving.
The term antenna gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. Gain is a key performance figure which combines the antenna's directivity and electrical efficiency. Antenna gain is usually defined as the ratio of the power produced by the antenna from a far-field source on the antenna's beam axis to the power produced by a hypothetical lossless isotropic antenna, which is equally sensitive to signals from all directions. Usually this ratio is expressed in decibels, and these units are referred to as “decibels-isotropic” (dBi). An alternate definition compares the antenna to the power received by a lossless half-wave dipole antenna, in which case the units are written as dBd.
Antenna gain is sometimes referred to as a function of angle, but when a single number is quoted the gain is the ‘peak gain’ over all directions.
Directivity measures how much more intensely the antenna radiates in its preferred direction than a mythical “isotropic radiator” when fed with the same total power. It follows then that the higher the gain of an antenna the smaller the effective angle of use. This directly impacts the choice of the antenna for a specific function. To achieve a directivity which is significantly greater than unity, the antenna size needs to be much larger than the wavelength. This can usually achieved using a phased array of half-wave or full-wave antennas. Since a phased array is comprised of a number of individual physically separate antennas, a phased array is not an adequate solution for particular mobile wireless communications due to the size of the aggregated individual antennas plus the gap distance between them.
An antenna radiation pattern is a graphical representation of the intensity of the radiation versus the angle from a perpendicular to a plane of the antenna. The graph is usually circular, the intensity indicated by the distance from the centre based in the corresponding angle. The radiation pattern may be used to determine the beamwidth which is generally accepted as the angle between the two points (on the same plane) at which the radiation falls to “half power” i.e. 3 dB below the point of maximum radiation.
Antenna impedance relates the voltage to the current at the input (feed port) to the antenna. The real part of the antenna impedance represents power that is either radiated away or absorbed within the antenna. The imaginary part of the impedance represents power that is stored in the near field of the antenna. This is non-radiated power. An antenna with only a real part input impedance (zero imaginary part) is said to be resonant. Note that the impedance of an antenna will vary with frequency. A common measure of how well matched the antenna is to the feed line (transmission line) or receiver is known as the Voltage Standing Wave Ratio (VSWR). VSWR is a real number that is always greater than or equal to 1. A VSWR of 1 indicates no mismatch loss (the antenna is perfectly matched to the transmission line). Higher values of VSWR indicate more mismatch loss.
Although a resonant antenna has by definition an almost purely resistive feed-point impedance at a particular frequency, many (if not most) applications require using an antenna over a range of frequencies. An antenna's bandwidth specifies the range of frequencies over which its performance does not suffer due to a poor impedance match. Bandwidth is typically quoted in terms of VSWR. For instance, an antenna may be described as operating at 100-400 MHz with a VSWR<1.5. This statement implies that the reflection coefficient is less than 0.2 across the quoted frequency range. Hence, of the power delivered to the antenna, only 4% of the power is reflected back to the transmitter. Alternatively, a return loss S11=20*log 10(0.2)=−13.98 dB. Note that the above does not imply that 96% of the power delivered to the antenna is transmitted in the form of electromagnetic radiation; losses must still be taken into account.
Dipole antenna conductors have the lowest feed-point impedance at the resonant frequency where they are just under ¼ wavelength long. The reason a dipole antenna is used at the resonant frequency is not that the ability of a resonant antenna to transmit (or receive) fails at frequencies far from the resonant frequency but has to do with the impedance match between the antenna and the transmitter or receiver (and its transmission line). Also in a half wave dipole antenna there is a natural peak in current distribution when fed at the centre. This type of antenna consists of two quarter wavelength sections fed exactly at the centre, where the wavelength lambda=c/f times the velocity factor of the dielectric medium surrounding the antenna, e.g. in the case of air, the velocity factor is approximately 0.95, which makes each section slightly less than a quarter wavelength (c=speed of light and f=resonant frequency).
As mentioned earlier, higher the gain of an antenna the smaller the effective angle of use. This directly impacts the choice of the antenna for a specific function. In mobile cellular applications the factors discussed above play an important consideration in trying to realize a small form factor efficient antenna.
Mobile devices more commonly have to operate on more than one frequency band, typically different portions of frequency spectrum thus requiring antenna designs that support multiband operation. In a conventional antenna design that supports multiband operation, a single broadband antenna has a single antenna port (feed point) connected to a single pole switch with multiple throws each connecting to different filter or duplexer units. Typically these filters incur losses of 0.5-0.7 dB when measured in a 500 system. In addition the switches also consume power, add a degree of non-linearity and have losses of 0.3-0.5 dB. Greater losses may be expected when the switches and diplexing networks are connected to an antenna due to inevitable mismatch.
With the deployment of LTE bands that currently extend towards the 700 MHz frequency and the upcoming deployment of LTE-A with Carrier Aggregation (CA), one can expect the need for a greater number of throws in the antenna switch for connecting to a larger number of filtering units. This imposes further challenges and potentially a need for additional antennas; especially if a single device for worldwide usage is to be designed as not all countries use the same frequency bands.
In a single port, multi-band antenna having multiple resonant frequencies generally leads to antenna design complexities. Single port multiband antennas are difficult to tune effectively for operation over the desired multiple frequency bands. For example, it is possible to obtain a dual-band antenna by choosing locations of varactors appropriately along the antenna so that first and second resonant frequencies can be controlled individually. In other words, the frequency of either the first or the second band can be fixed, while the other one is electronically tuned.
On the other hand, a multi-band antenna having multiple antenna feed points (multiport) tends to reduce antenna design complexities since the design of a plurality of individual radiating/receiving elements, each having a separate feed, tends to be less difficult. However, multiple antenna feeds encounter the problem of mutual coupling between the individual radiating/receiving elements of a multi-band antenna. There is also a concern that a multi-band antenna with multiple antenna feed ports may have its performance compromised due to mutual coupling and poor isolation between the antennas various resonant bands. For example dual-feed, dual-band, PIFAs have been used for cellular mobile wireless applications. However, most of these dual-feed, dual-band, PIFAs exhibit an isolation of only about 15 dB, degraded gain at the individual antenna ports. And employ both physical and electrical separation between the radiating/receiving elements which also involves a change in the linear dimensions of the separate radiating elements resulting in increased overall physical volume